1. Field of the Invention
The present invention relates to an sound processing apparatus and, in particular, to a sound processing apparatus for amplifying a sound signal input from a microphone and processing the sound signal, a gain control apparatus for controlling the amplification gain of the sound signal, a gain control method for use in the sound processing apparatus and the gain control apparatus, and a program product storing a computer program for executing a program for the sound processing apparatus and the gain control apparatus.
2. Description of the Related Art
In recent years, the use of apparatuses has become widespread that receive sound with a microphone, convert the sound into a sound signal, amplify the sound signal to an audio level appropriate for speech recognition processing, and performs speech recognition processing on the speech of a speaker on the basis of the amplified sound signal. When the audio level (power) is excessively increased, the sound signal is saturated. In contrast, an excessively decreased audio level results in erroneous speech recognition. Accordingly, in order to obtain an appropriate audio level, it is important to develop a technology for controlling the amplification gain.
FIG. 22 is a functional block diagram of an existing sound processing apparatus 1000. The existing sound processing apparatus 1000 includes a sound receiving unit 1010 for receiving a sound arriving from the outside, such as speech uttered by a speaker and noise, an amplifier unit 1020, such as a variable gain amplifier, for amplifying a sound signal generated by the sound receiving unit 1010, an analog-to-digital (A/D) converter 1030 for converting the analog sound signal amplified by the amplifier unit 1020, and a control unit 1040, such as a digital signal processor (DSP).
The control unit 1040 includes speech section detecting part 1041 for detecting a section that contains speech uttered by a speaker from the sound signal converted to a digital signal and application part 1042 for executing a speech recognition process on the basis of the speech contained in the detected section. The control unit 1040 further includes power estimating part 1043 for estimating an audio level (power) on the basis of the sound signal in the detected section and gain deriving part 1044 for computing a control value of the gain of the amplifier unit 1020 on the basis of the estimated audio level. The computed gain control value is output to the amplifier unit 1020, which amplifies the sound signal on the basis of the gain control value.
By controlling the gain for amplifying the sound signal in the amplifier unit 1020 so as to adjust the sensitivity of the sound receiving unit 1010, the audio level of the sound signal required for executing a speech recognition application program can be made close to a target value under various noise environments. This feature is referred to as “automatic gain control (AGC)”. The AGC is used for pre-processing when an application such as a speech recognition application program is executed.
Japanese Laid-open Patent Publication No. 2000-105599 describes a method for separating noise from speech in a signal containing the noise and the speech, as one of technologies relating to sound processing.